You may think you’re at perfect peace when you sleep, but the truth is that your brain is active. In fact, your brain and your body move in and out of several different stages of sleep through the night. Each has different characteristics, so scientists can always tell what stage a person is in, especially if the sleeper’s brain waves are being monitored.

Altogether, there are five stages of sleep. Four of these, Stages 1-4, are non-REM sleep. The last one is REM sleep. Most people cycle through these throughout the night, about once every 90 minutes. However, you spend more time in different stages at different times of night, and each person’s sleep cycles are slightly different.

Want to learn more? Here are the stages we all go through when we sleep.

Stage 1

This is the lightest stage of sleep. You can drift in and out of stage 1 sleep and may have the sensation that you can’t possibly keep your eyes open any longer. You are basically transitioning into true sleep.

In Stage 1, your body is beginning to slow down. Your brain waves are slightly slower than they are when you’re awake, but otherwise not much has changed there. Your respiration rate and heart rate will begin to slow down, though your body temperature remains the same as it was before.

The body’s muscles retain some tension in Stage 1, though they begin to relax. If you have ever experienced the sensation of falling and twitched yourself awake, that happens in Stage 1 sleep. Stage 1 usually lasts for 10 minutes or less, and it’s easy to wake up from this sleep. In fact, many people woken during Stage 1 don’t think that they actually slept at all.

Stage 2

Stage 2 sleep is a continuation of Stage 1. The brain waves slow significantly, though they usually show some instances of heightened, flurried activity. Eye movement ceases, and the heart and lungs slow even more. The body’s temperature also begins to fall during Stage 2, further preparing for deep sleep.

During Stage 2, most people are still fairly easy to waken. They can answer the phone straight out of sleep and still be coherent enough that the person on the other end may not be able to guess that they were sleeping at all.

You spend most of your night in Stage 2 sleep, passing through it every time you move from lighter to heavier sleep and back again. In fact, some people spend as much as half the night here.

Stages 3 & 4

 

 

Stages 3 and 4 together make up deep sleep. The brain waves here have slowed significantly from their usual daytime patterns, forming the long, slow waves that scientists call delta waves. In Stage 3, these waves can be broken up by smaller, faster waves, while Stage 4 is characterized exclusively by delta waves.

In these stages, your heart and respiration rates have slowed as much as they’re going to, which is usually a significant change from where they are when you’re awake.

People who wake up during these stages of sleep often become groggy and even confused. They may be very hard to rouse, requiring you to touch them or shake them rather than just talk to them. These are the stages where people experience night terrors, sleepwalking, and more.

REM Sleep

REM sleep is different from the other stages of sleep in that the brain becomes more active, sometimes as active as it is during the day. However, the body is paralyzed during this sleep cycle. It’s probably a good thing since we tend to dream while we’re in REM sleep. The paralysis keeps us from acting out all of our dreams!

The rest of the body returns to a near-normal state, too. Both the heart rate and the respiration rate pick up again, and eye movement returns, too. Researchers aren’t sure exactly what the eyes are doing during REM sleep, though they posit that they may be moving along with a dream.

People who struggle with sleep apnea or other sleep disorders can struggle during REM sleep. Because their muscles are paralyzed, their airways can collapse completely. Many of them have trouble breathing after this happens, as they can’t fully restore the airways while asleep.

Most people experience more REM sleep as it gets closer to morning. This might be the reason why so many of us awaken straight from a dream.

Sleep is fascinating and more complex than most of us think. The more we begin to understand about when, how, and why we sleep, as well as what goes on while we sleep, the better we can become at making sure we are all getting the sleep we need.